1. Field of the Invention
The present invention relates to a device for the detection and discrimination of operational faults in an electrical power supply and, more particularly, to such a device which is adapted to a power supply of a load connected in series with an electronic switch controlled by a computer.
2. Description of the Prior Art
At present, in the automotive industry for example, electronic computers are used for controlling various actuators, injectors, electromechanical valves, ignition coils, electric motors, etc. By way of non-limitative example the electromechanical valves of a motor vehicle wheel anti-locking device may be considered such a controlled element. FIG. 1 of the drawings schematically illustrates the electronic section of a known device of this type for controlling six electromechanical valves EV-0-EV-5 used for modulating the pressure in the braking circuits of wheels after detection of the imminence of skidding of at least one of the wheels of the vehicle with respect to the ground. The excitation winding of each electromechanical valve is connected, on the one hand, to a voltage source +V.sub.BAT constituted by the battery of the vehicle and, on the other hand, to an OUT pin of a control circuit (CC-0-C-5) for the power supply of the winding by a selective winding to ground GND. In order to do this, the circuit conventionally comprises a power transistor, of the metal-oxide-semiconductor (MOS) type for example, whose drain-source circuit is connected between the OUT pin and the ground pin GND. The switching of the power transistor is ensured by signals received from a main computer and supplied to the input IN of the control circuit, via an input bus. Such control circuits are well known in the art and normally include, in addition to a power section in which the current coming from the battery flows, a section powered at a low current and capable of processing logic signals used, on the one hand, for controlling the power transistor and, on the other hand, for the detection of operational faults which are signaled to the main computer by signals emitted on a "STATUS" pin and transmitted to the computer by way of a "status" bus. Such a control circuit is commonly called a "smart power circuit" as it is capable both of controlling an actuator using its power section and of processing logic signals, for example, for the detection of short circuits or other faults capable of preventing the correct operation of an electromechanical valve and, more generally, of the entirety of the device. Furthermore, the anti-locking device can also comprise an auxiliary monitoring computer which executes redundant computations on the signals received by the main computer in order to check the correct operation of the main computer. A power supply voltage V.sub.CC, for example +5 volts, supplies each of the control circuits and computers and return load resistors connected to the inputs IN-0-IN-5 of the main computer connected to the status bus and to lines interconnecting the pins 0-5 of the two computers in order to provide for dialogue data exchanges therebetween.
In the above-mentioned application to the control of an anti-lock system of the wheels of a motor vehicle, it is very important to know the operational state of each of the electromechanical valves as an operational fault of one or more of such valves can seriously affect the safety of the driving of the vehicle. It is observed, however, in the diagram of the known device illustrated in FIG. 1, that the transmission to the computer of data generated by the control circuits CC-0-CC-5, relating to possible operational faults in the circuits or in the electromechanical valves associated therewith, requires that a rather large number of input and output pins (or "ports") are available on the computers. In addition to six output pins OUT-0-OUT-5, used for sending to each control circuit commands for switching power transistors, each computer must comprise input pins IN-0-IN-5 in order to receive data relating to possible operational faults in the control circuits. Unfortunately, the microprocessors currently available for equipping devices, of the type illustrated in FIG. 1 comprise a limited number of input/output pins while it is desired to increase the number of actuators capable of being controlled by one microprocessor. It therefore appears that this increase can only be made possible by a reduction in the number of lines used for exchanging data between each control circuit and the associated computers, while retaining all the functions mentioned above. Furthermore, the known control circuits, after detection of an operational fault (thermal overload, short circuit load, open circuit load, etc) are limited to maintaining the STATUS line at a logical "0", a measure which does not permit the computer to determine the nature of the fault. This is a substantial disadvantage as all of the operational faults are not equally important and it would be useful for the operation in a degraded mode of the control device, established by the computer on receiving a fault signal, to be able to take account both of the nature of the fault and of the position of the electromechanical valve which is affected thereby. By way of example, a permanent control or excitation of an electromechanical valve due to an internal or external short circuit of the power transistor of the control circuit associated with the electromechanical valve can give rise to a drop in pressure in the hydraulic circuit controlling a wheel brake, which dangerously affects the driver's ability to stop his vehicle. The degraded operation established by the computer should be able to take account of the dangerous nature of such a failure, in order to still ensure the safety of the braking.